The greatest remaining clinical problem in Lyme disease (LD) is that some patients do not have complete resolution of signs or symptoms with antibiotic therapy. In recent years, we have studied this problem intensively in patients with Lyme arthritis in whom synovitis persists for months or years after spirochetal killing with antibiotics, called antibiotic-refractory arthritis. We have hypothesized that this disease course results from infection-induced autoimmunity with excessive inflammation and immune dysregulation in joints. However, until recently, identification of pathogenic autoantigens has remained elusive. Using discovery-based proteomics combined with clinical translational research, we recently identified endothelial cell growth factor (ECGF), the first known autoantigen that induces T and B cell responses in some patients with LD. In addition, we have preliminary information that the third component of complement and cytokeratin 10 may also serve as autoantigens. Our first specific aim in this grant application is to determine the clinica correlates of autoimmunity to ECGF or other candidate autoantigens across the spectrum of LD, including its post-infectious syndromes, to assess whether these responses are specific for LD, and to learn whether they occur only with U.S. B. burgdorferi (Bb) strains or also with European genospecies of the spirochete. Second, we plan to use the same proteomic and translational research approach that we employed successfully in the identification of ECGF to identify other candidate autoantigens. Third, we will determine spirochetal and host factors, including types of Bb strains and interactions with host cells, which are necessary for the induction of immune responses to ECGF of other candidate autoantigens, and we will characterize the adaptive immune response to these autoantigens in target tissues or fluids. These aims will allow us to determine whether autoimmunity to ECGF occurs as a part of a wider spectrum of Bb-induced immune responses, to ascertain the resulting clinical pictures, to identify other candidate autoantigens, and to delineate mechanisms responsible for these responses. We anticipate that a test for these antibody responses will become a part of the clinical evaluation of LD. Such information will help physicians to treat patients with LD more appropriately and effectively.